In recent years, development of a power generation system using a solar cell and a fuel cell having little influence on the environment is being positively progressed from the viewpoint of global environmental protection. In this power generation system, a power converting apparatus for system connection as an inverter device converts generated direct power into alternate-current power corresponding to frequency and voltage of a commercial power system, and supplies this alternate-current power to the commercial power system (that is, converted into an adverse current). An outline of a power converting apparatus for system connection that is conventionally used when a single phase is a commercial power system as a grounding wire is explained below with reference to FIG. 10 to FIG. 15. A case that a direct current voltage generated by the direct-current power generation facility is higher than a system voltage (wave crest value), that is, when a booster circuit is not used, is explained.
FIG. 10 is a block diagram of a configuration example (part one) of a conventional power converting apparatus for system connection. FIG. 11 is a relevant waveform diagram for explaining the operation of the power converting apparatus for system connection shown in FIG. 10. FIG. 10 is an example of a configuration of a connection inverter using a full bridge. Namely, in FIG. 10, when a direct-current power source as a direct-current power generation facility is a solar cell, grounded capacitance (stray capacitance) is present between a positive electrode side and the earth and between a negative electrode side and the earth, respectively. A system power source U is a commercial power system having one phase as a grounding wire. In this commercial power system, a grounded phase Ug of the system power source U is grounded, and a grounding wire of the grounded phase Ug is wired to a consumer, together with the power supply wire of a non-grounded phase Ua.
A high-voltage bus line P is connected to a positive electrode (voltage Vi+) of the direct-current power source E, and a low-voltage bus line N is connected to a negative electrode (voltage Vi−). A smoothing capacitor C1 is connected to between the high-voltage bus line P and the low-voltage bus line N. Two sets “S1, S2” and “S3, S4” of two switching elements connected in series are connected in parallel between the high-voltage bus line P and the low-voltage bus line N. A backflow diode D is connected inversely parallel to each switching element.
The switching elements S1 to S4 are IGBTs (Insulated Gate Bipolar Transistors), for example, and are on/off driven by a PWM signal from an operation control circuit not shown. A total of the switching elements S1 to S4 and the operation control circuit not shown that drive controls these switching elements S1 to S4 is an inverter controller.
One end of a reactor L1 is connected to an end of the series connection between the switching elements S1, S2 at the system power source U side. The other end of the reactor L1 is connected to one end of a capacitor C2 and the non-grounded phase Ua of the system power source U. One end of a reactor L2 is connected to an end of the series connection (inverter output side) between the switching elements S3, S4 at the direct-current power source E side. The other end of the reactor L2 is connected to the other end of the capacitor C2 and the grounded phase Ug of the system power source U. The reactor L1, the capacitor C2, the reactor L2, and the capacitor C2 constitute a low-pass filter.
In FIG. 11, a non-grounded phase system voltage (expressed as “Ua”) is a sinusoidal wave vertically symmetrical based on a grounded phase system voltage (expressed as “Ug”). Among the switching elements S1 to S4 of the inverter controller, a set of the switching elements “S1, S4” and a set of the switching elements “S2, S3” alternately carry out an on/off operation based on a predetermined PWM signal in synchronism with a positive half cycle and a negative half cycle of the non-grounded phase system voltage Ua, thereby switching a voltage between bus lines. As a result, an alternate current voltage as a smooth sinusoidal wave similar to the non-grounded phase system voltage Ua is generated in low-pass filters “L1, C2” and “L2, C2”, and is output toward the system.
In this case, the inverter controller operates based on the end of the series connection between the switching elements S3, S4 at the direct-current power source E side. Because the system voltage varies, the output of the inverter controller also varies, and the voltage between bus lines changes. The end of the series connection between the switching elements S3, S4 is a virtual intermediate point of the direct-current power source E. Therefore, based on the grounded phase Ug of the system power source U, the virtual intermediate point of the direct-current power source E varies by only the inverter output. This means that the ground voltage of the direct-current power source E varies in the cycle of the system.
In this case, when the direct-current power source E has a large grounded capacitance (stray capacitance) such as the capacitance of a solar cell, the following problems arise. First, when the direct-current power source E has a grounded capacitance, an alternate current (leak current) that is charged to and discharged from the electrostatic capacitance flows in the route of the backflow diode D to the stray capacitance of the solar cell to the earth, connected to each switching element of the system to the inverter controller.
Usually, a leak detector and a leak breaker are installed between the low-pass filters “L1, C2” and “L2, C2”. When a detected leak current exceeds a predetermined value, the leak breaker carries out a break operation to separate the corresponding power converting apparatus for connection from the system. Therefore, when the direct-current power source E is a solar cell, the leak breaker operates due to a large grounded capacitance. Because the solar cell itself is exposed to rain and snow, when the grounded capacitance increases along the increase of wet positions, a leak current increases accordingly. As a result, the above break operation of the leak breaker occurs easily.
When the ground construction is insufficient, a high voltage is accumulated in the stray capacitance. Therefore, there is a risk that this stray capacitance harms a person when this person touches the stray capacitance.
FIG. 12 is a block diagram of a configuration example (part two) of a conventional power converting apparatus for system connection. FIG. 13 is a relevant waveform diagram for explaining the operation of the power converting apparatus for system connection shown in FIG. 12. FIG. 12 is an example of a configuration of a connection inverter using a half bridge. In other words, in FIG. 12, series-connected two capacitors C3, C4 are connected between a high-voltage bus line P and a low-voltage bus line N. Series-connected two switching elements “S1, S2” are also connected between the high-voltage bus line P and the low-voltage bus line N. The two capacitors C3, C4 have equal capacitances.
One end of a reactor L3 is connected to an end of the series connection (inverter output end) between the switching elements S1, S2. The other end of the reactor L3 is connected to one end of a capacitor C5 and the non-grounded phase Ua of the system power source U. The other end of the capacitor C5 and the grounded phase Ug of the system power source U are connected to the end of the series connection between the two capacitors C3, C4. The reactor L3 and the capacitor C5 constitute a low-pass filter.
The switching elements S1, S2 of the inverter controller alternately carry out an on/off operation based on a predetermined PWM signal in synchronism with a positive half cycle and a negative half cycle of the non-grounded phase system voltage Ua, thereby switching a voltage between bus lines. Low-pass filters “L3, C5” remove a high-frequency component. As a result, an alternate current voltage as a smooth sinusoidal wave similar to the non-grounded phase system voltage Ua is generated, and is output toward the system.
In this case, the inverter controller operates based on one end of the series connection between the switching elements S1, S2. Because the series-connected capacitors C1, C2 equally bear the voltage of the direct-current power source E, a potential of the end of the series connection between the capacitors C1, C2 is an intermediate voltage of the direct-current power source E. Therefore, in the configuration that has the end of the series connection between the capacitors C1, C2 connected to the grounded phase Ug of the system power source U, the inter-bus line voltage is stable even when the voltage of the system varies. The inverter output end outputs a voltage equivalent to the non-grounded phase system voltage Ua.
As a result, the ground voltage of the direct-current power source E becomes a direct current voltage. Therefore, even when the grounded capacitance is present in the direct-current power source E, the above leak current is prevented from flowing, and there is no risk that the leak breaker carries out the break operation. However, in the connection inverter of a half bridge, positive and negative wave crest values of the system voltage need to be output. Therefore, the voltage of the direct-current power source E needs to be a substantially larger value than the positive and negative wave crest values of the system voltage. For example, when the system power source is AC 230V, the voltage of the direct-current power source E needs to be equal to or larger than “2√2” times of AC 230V, that is, DC 630V. In other words, the voltage of the direct-current power source E of the connection inverter of a half bridge needs to be two times of the direct-current power source E of the full bridge connection inverter shown in FIG. 10. A withstand voltage of a used switching element also needs to be two times. In general, a switching element having a high withstand voltage has larger power loss. Therefore, the power conversion efficiency of the connection inverter of a half bridge decreases significantly.
FIG. 14 is a block diagram of a configuration example (part three) of a conventional power converting apparatus for system connection. FIG. 15 is a relevant waveform diagram for explaining the operation of the power converting apparatus for system connection shown in FIG. 14. In FIG. 14, a transformer T is provided in place of the reactors L1, L2 in the configuration shown in FIG. 10.
In this configuration, the operation of the inverter controller is similar to that of the inverter controller shown in FIG. 10. The transformer T insulates between the inverter controller and the system power source U. Therefore, even when a voltage of the system varies, a voltage between bus lines does not change, and the above leak current is prevented from flowing. However, because a power loss occurs due to the transformer T, the power conversion efficiency decreases significantly. Because the transformer T has a large mass and is heavy in general, it is difficult to decrease the size and weight of the apparatus.
As described above, according to the connection inverter of a half bridge, two series-connected capacitors equally bear the voltage of a direct-current power source, and give an intermediate voltage of the direct-current power source. Therefore, when a grounding wire of the system is connected to the end of the series connection, a leak current can be avoided even when the direct-current power source has a large grounded capacitance like that of a solar cell.
A three-level inverter is known as an inverter that uses two series-connected capacitors that equally bear the voltage of the direct-current power source. The three-level inverter is an inverter that switches between three voltage levels of a high voltage, a low voltage, and an intermediate voltage. The three-level inverter has a complex circuit configuration as compared with a two-level inverter that alternately output-controls a high voltage and 0V. However, the three-level inverter has advantages: (1) because the inverter can decrease a higher harmonic component, the inverter has little noise; (2) the inverter can minimize pulsation of a motor torque during a driving of a three-phase motor; (3) the three-phase motor has small magnetic distortion noise; and (4) the inverter can set a low withstand voltage of a switching element, and therefore, this three-level inverter is widely used. Patent Document 1, for example, discloses a technique of pulse-width modulating two carrier signals by switching the carrier signals, in a simple processing, depending on whether the voltage command value is larger than a predetermined voltage command value, to acquire an output voltage to meet the voltage command value of any amplitude, in driving a three-phase motor.
When a three-level inverter having the above excellent characteristics is used for system connection, a leak current can be avoided even when a direct-current power source having a large grounded capacitance like that of a solar cell is used. Further, because the three-level inverter has no transformer, the power conversion efficiency can be improved, and the size and weight of the system can be decreased.
Patent Document 1: Japanese Patent Application Laid-open No. H9-163755